It is recognised that forward osmosis desalination is a much less energy consuming process compared to reverse osmosis. However, the problem in the state of the art is the lack of a suitable draw solute to be used for forward osmosis. Some draw solutes used are able to generate high osmotic pressure but are unable to be regenerated. Other draw solutes being used may be easily regenerated, but are either unable to produce high osmotic pressure or potentially toxic to human health. A commonly used draw solute is ammonium bicarbonate. However, the drawbacks include high reverse diffusion. Further, while the ammonium bicarbonate can be easily recovered by heating up to 65° C., the draw solute dissociates into carbon dioxide and ammonium, and inevitably some ammonium remains in the water and influences the product water quality.
N,N-dimethylcyclohexylamine has also been studied as a potential draw solute. Although a high osmotic pressure can be generated with this draw solute, the problem of this draw solute is that there is membrane degradation, large reverse diffusion and a relatively tedious regeneration method.
Another example of a draw solute is 2-methylimidazole-based ionic liquid. The dissociation of ionic liquid increases the osmolality, but the small molecular weight and strong interaction with water make it difficult to separate from water. Polyelectrolyte of poly sodium acrylate has also been tested. The polyelectrolyte can also generate high osmotic pressure and its relatively larger molecule size compared with ionic liquid makes it possible to be isolated from water by ultrafiltration. However, high pressure (˜10 bar) is used and the membrane clogging is inevitable due to the high concentration (˜0.1 g/mL) of polymer solution. Other non-functional draw solutes such as glucose, inorganic or organic salts and polyelectrolytes always encounter the trade-off between high osmotic pressure and easy regeneration.
Therefore, they either are incorporated into the final product, or have to rely on some membrane separation process for their recovery, thereby resulting in membrane clogging and low water recovery.
There is therefore a need for an improved draw solute which is able to generate high osmotic pressure, be easily regenerated and cause minimal membrane degradation for use in forward osmosis.